Bipolar transistors also referred to as bipolar junction transistors (BJT) are transistor devices commonly employed with other circuit elements for voltage gain, current gain, or signal power gain. Bipolar transistors are semiconductor devices in which both electrons and holes participate in the conduction process. Some example uses for bipolar transistors include high speed circuits, analog circuits, and power applications.
One example of integration of bipolar transistors with complementary metal oxide semiconductor technology is referred to as bipolar complementary metal oxide semiconductor (BiCMOS) technology, which is an integrated circuit technology that combines bipolar devices with CMOS to yield integrated circuit devices that can have reduced noise, improved linearity, device matching, and high drive capacity, and permit improved performance. BiCMOS integrated circuits have bipolar and MOS transistors formed on the same semiconductor body or substrate. One advantage of such devices is that they combine the high power and fast switching speeds of bipolar devices with the high density and low power consumption of MOS transistors. The diversity of uses for such BiCMOS devices has fueled a surge toward fabricating faster, denser and more powerful integrated BiCMOS devices by more individual device enhancing manufacturing processes.
Bipolar junction transistors consist of three differently doped semiconductor regions, an emitter region, a base region and a collector region. In operation, current flows between the emitter and the collector according to relatively small changes in the voltage applied across the base-emitter regions. Bipolar transistors differ from other types of transistors in that electrode contacts are typically located at a single top or upper surface of devices. As a result, isolation regions can be required in order to isolate devices and permit proper operation of bipolar devices without interference from surrounding devices.
In order to reduce series resistance of the collector, a buried layer, generally n-type, is present and is located below base, emitter, and collector regions of fabricated bipolar transistor devices. Heavy doped buried layer gives better isolation to substrate which is widely used in BiCMOS technology. Conventional fabrication techniques for forming buried layers generally form a thick hard mask and then implant a selected dopant. Then, a drive in or diffusion process is performed to diffuse the implanted dopants. However, the conventional buried layer fabrication techniques typically consume a relatively large amount of underlying silicon. As a result, a silicon recess is generally formed that can be problematic for subsequent fabrication processes, limit the types of isolation regions employed, and/or hamper formation of isolation regions.